Direct current motors are known in which a plurality of magnets are positioned with their polar axes extending generally radially with respect to the axis of spin of the motor. Such motors are designed to use either electromagnets or permanent magnets for the field structure. A common feature of all such motors has been the provision of a ferromagnetic shell around the outside of the motor through which the flux of all the outwardly directed poles may pass. Various pole piece configurations have been developed, particularly for use with permanent magnets.
Motors which utilize permanent magnets such a Alnico 5, which has comparatively low intrinsic coercivity, often utilize soft iron platelets or pole shoes to prevent partial demagnetization which could otherwise occur under heavy load or reversal conditions. With permanent magnets of the barium or strontium ferrite family and also the samarium cobalt family, the added pole shoes are sometimes combined with poles in unusual configurations to achieve a higher rotor gap flux density than exists at the magnet surface. This increased rotor gap flux density is a direct motor performance factor for increased torque per ampere ratio, and is most advantageous with samarium cobalt magnets having high energy, high coercivity, but only moderately high residual flux density.
Soft iron pole shoes typically offer a further advantage during fabrication where finishing to a critical radius and concentricity is required, preferably by machining instead of grinding the hard and brittle magnet material.
The typical incorporation of soft iron pole shoes is represented in structures as shown in U.S. Pat. Nos. 3,296,471, 3,564,705 and 3,828,213 for example. The surrounding cylinder or box of ferromagnetic material serves to support the pole and magnet structure but also provides outer flux flow return between poles of opposite polarity. The thickness of the outer shell is determined by the maximum allowable flux density which occurs in the cross section between opposite poles. Much of the remainder of the outer shell operates at lower flux density and magnetic efficiency and therefore contributes to excessive weight and size for a given armature diameter. Moreover, such structures have traditionally necessitated a final grinding operation during manufacture to true up the rotor cavity, to insure the minimal required air-gap clearance.